This invention relates generally to optical waveguides, and more particularly to optical waveguides having a core containing a siloxane polymer and a method for fabricating the waveguides.
As devices and interconnects used in integrated circuits continue to decrease in size, the speed with which information can be encoded and sent through the circuit by way of interconnects has become a significant factor in determining the ultimate speed of the integrated circuit itself. To increase these interconnect speeds, optical components can be incorporated into the integrated circuit or microprocessor.
In an optical or photonic interconnect system, an electrical signal is converted into an optical signal in one portion of a circuit, transmitted to another part of the circuit several millimeters or centimeters away, and converted back into an electrical signal at the new location. To operate, optical interconnects require sources, modulators, a transmission medium, and receivers. Optical interconnects on the multichip module, chip-to-chip, or chip-to-board level also involve integrated passive waveguides, light-steering components, and active optical devices, such as active optical waveguides. With respect to active optical waveguide architectures, light propagates parallel to the substrate surface in planar waveguide structures and perpendicular to the surface in stacked structures.
In planar waveguide designs, active optical waveguides include a core and a cladding material contacting and partially or entirely surrounding the core. In addition, the core material must have a higher refractive index than the cladding. For an electrical/photonic scheme, an optical waveguide having a thin core and cladding (xe2x89xa610, but preferably less than 5 xcexcm) and small bending radii ( less than 10 xcexcm) is most desirable. A thin core and cladding allow small inter-waveguide spacing for a given amount of crosstalk. In addition, tighter turns for a given value of radiation loss can be achieved by having a high refractive index contrast (xcex94n) between the core and the cladding of the waveguide. Applications for high refractive index contrast waveguide systems include dense on-chip interconnects and other VLSI photonics components such as micro-ring resonators.
Because integrated circuits are conventionally silicon-based materials, optical waveguides for use therein must be fully compatible with silicon processing at the back end of the line and/or with other steps used in processing these circuits. Desirable materials should exhibit the following properties: 1) good adhesion to interconnect metals, e.g., copper and aluminum, using, if necessary, adhesion promoters and/or buffer layers; 2) good adhesion to silicon and silicon oxide; 3) good thermal stability at temperatures up to 350xc2x0 C. for short processing times (e.g., 30 minutes) and 150xc2x0 C. for long periods (e.g., several hours); and 4) the capability of being processed into waveguides and couplers using techniques, chemicals, temperatures, and other conditions that are compatible with silicon chip processing. The materials should also be stable during phase transitions (i.e. crystallization or melting), be chemically stable, and be stable in terms of optical loss, index of refraction, and density.
In addition to being compatible with silicon circuitry, optical waveguides should meet the demands of low optical loss, as well as meet the requirements of an electronic environment. For example, for use in on-chip applications, it is desirable that the optical loss at each wavelength of interest be less than 1 dB/cm in a 2 micron by 2 micron cross-section straight waveguide.
Particular wavelengths of interest for information transfer generally lie from about 400 nm to about 2000 m for specialty (sensor) applications. Certain useful wavelengths include, e.g., 635 nm, 840 nm, 900 nm, 1300 nm, and 1550 nm, with the longer wavelengths corresponding to common communications systems.
As mentioned above, to be of significant use, optical waveguides should have a high refractive index contrast (xcex94n) between the waveguide core and the cladding. The difference in refractive indices must be at least 0.02, but is more preferably 0.05 or higher.
The waveguides of the present invention exhibit unexpectedly low optical losses and meet the aforementioned compatibility needs and the refractive index contrast requirements.
It should be noted that variables are defined when introduced and retain that definition throughout.
In one aspect, the present invention relates to optical waveguide structures comprising a light-transmitting core material having a first refractive index, and a cladding material contacting and partially or entirely surrounding the core material. The cladding material has a second refractive index lower than the first refractive index of the core material. The core material is a siloxane resin composition comprising:
(A) from about 95 to about 100 parts by weight of a siloxane polymer comprising structural units having the formulae X and Y: 
and terminating in residues OR8 and R8, wherein
(1) FG is a functional group, and each FG in said polymer is independently chosen from
(a) linear, branched, and cyclic alkyl residues of 1 to 20 carbons terminating in a 1-alkenyl ether;
(b) linear, branched, and cyclic alkyl ether residues of 1 to 20 carbons and 1 to 9 oxygens terminating in a 1-alkenyl ether;
(c) linear, branched, and cyclic alkyl residues of 1 to 20 carbons terminating in an acrylate, an alpha-chloroacrylate, an alpha-cyanoacrylate, or a methacrylate;
(d) linear, branched, and cyclic alkyl ether residues of 1 to 20 carbons and 1 to 9 oxygens terminating in an acrylate, an alpha-chloroacrylate, an alpha-cyanoacrylate, or a methacrylate;
(e) linear, branched, and cyclic alkyl residues of 1 to 20 carbons substituted with an epoxide;
(f) linear, branched, and cyclic alkyl ether residues of 1 to 20 carbons and 1 to 9 oxygens substituted with an epoxide;
(g) arylalkyl residues of 1 to 20 carbons substituted with an epoxide;
(h) arylalkyl ether residues of 1 to 20 carbons and 1 to 9 oxygens substituted with an epoxide; and
(i) epoxy-functional organosiloxane residues of 1 to 20 silicons and 1 to 20 carbons;
(2) R is alkyl, aryl, haloalkyl, or aralkyl of 1 to 10 carbons;
(3) R1 is R, 
(4) R2 is alkyl, aryl, haloalkyl or aralkyl of 1 to 10 carbons or 
(5) R3 and R4 are independently alkyl, aryl, haloalkyl, aralkyl, alkoxy or aryloxy of 1 to 10 carbons;
(6) R5, R6 and R7 are independently FG, alkyl, aryl, haloalkyl, aralkyl, alkoxy or aryloxy of 1 to 10 carbons;
(7) R8 is alkyl, aryl, haloalkyl, or aralkyl of 1 to 10 carbons;
(8) m and n are each independently 2 to 50;
(9) p is 2 to 50; and
(10) q is 0 to 50; and
(B) from 0 to about 5 parts by weight of a polymerization initiator selected from the group consisting of free radical initiators and cationic initiators selected from the group consisting of diazonium, sulfonium, phosphonium, and iodonium salts, wherein said selected cationic initiator is present in a catalyst solution comprising from about 20 to about 60 parts by weight of the selected cationic initiator and from about 40 to about 80 parts by weight of 3,4-epoxycyclohexylmethyl-3xe2x80x2,4xe2x80x2-epoxycyclohexane carboxylate, dicyclopentadiene dioxide, or bis(3,4-epoxycyclohexyl) adipate.
Preferred cladding materials include siloxanes and fluorinated siloxanes having a lower refractive index than the core, as described below, silica xerogels (i.e., porous silicon oxide), silicon oxide, metal oxides, air, silicon dioxide, benzocyclobutene, plasma oxides, acrylates, fluorinated acrylates, polyimides, and other polymers having a lower refractive index than the core.
In another aspect, the present invention relates to optical waveguide structures comprising:
[1] a light-transmitting core material comprising a first siloxane resin composition having a first refractive index, wherein the first siloxane resin composition comprises:
[1] from about 95 to about 100 parts by weight of a first siloxane polymer;
(2) from about 0 to about 5 parts by weight of a first polymerization initiator independently selected from those previously described; and
(B) a cladding material comprising a second siloxane resin composition having a second refractive index lower than the first refractive index of the first siloxane resin composition. The cladding material contacts and partially or entirely surrounds the core material. The second siloxane resin composition comprises:
(1) from about 95 to about 100 parts by weight of a second siloxane polymer;
(2 from about 0 to about 5 parts by weight of a second polymerization initiator, independently selected from those previously described.
The first and second siloxane polymers each contain independently selected structural units X and optionally, Y, and terminate in residues OR8 and R8, as described above. Each FG, R, R1, R3, R4, R5, R6, R7, R8, m, n, p, and q of the first siloxane polymer is selected independently from that of the second siloxane polymer.
In another aspect, the present invention relates to a method for fabricating an optical waveguide structure. The method includes the following steps:
(1) providing a substrate;
(2) forming a first layer of a cladding material over the substrate, wherein the cladding material has a second refractive index;
(3) depositing atop the first layer of cladding material a core layer comprising
(A) from 0 to about 95 wt. % solvent; and
(B) from about 5 to about 100 wt. % of a core siloxane resin composition comprising:
(I) from about 95 to about 100 parts by weight of a core siloxane polymer, wherein said core siloxane polymer comprises structural units having formulae X and Y and terminating in residues OR8 and R8, as previously described; and
(II) from 0 to about 5 parts by weight of a core polymerization initiator, as previously described, and
(4) curing the core layer thermally, or using actinic or e-beam radiation to form a light-transmitting core material having a first refractive index higher than said second refractive index of said cladding material.
Optionally, after step (4), the method can also include forming a second layer of cladding material atop the light-transmitting core material. In addition, prior to adding the second layer of cladding material, the light-transmitting core material can be patterned, such that a portion of the first layer of cladding material is free of the light-transmitting core. Also, when actinic radiation is used to cure the core layer, patterning may be performed simultaneously with curing. Alternatively, between steps (3) and (4), the core layer can be patterned before curing.
In another aspect, the method may include the step of forming a groove in the first layer of cladding material between steps (2) and (3). However, the substrate remains covered with the first cladding layer. Then, the core layer is also deposited into the groove. After curing, the light-transmitting core material overlying the first layer of cladding material may be removed, but the light-transmitting core material remains in the groove. A second layer of cladding material can be formed atop the first layer of cladding material and atop the light-transmitting core material in the groove.